1. Field of the Invention
The present invention relates to a single-cylinder 4-cycle engine including a control means for feedback-controlling the amount of fuel supplied, based on a signal from an oxygen concentration sensor for detecting a concentration of oxygen in an exhaust gas. The concentration sensor is provided between an exhaust port and an exhaust emission control catalyst.
2. Description of the Prior Art
In an engine including an exhaust emission control catalyst of a noble metal such as platinum-rhodium and the like, disposed in an exhaust passage, to convert harmful components in an exhaust gas, it is a conventional practice to detect the concentration of oxygen in the exhaust gas with an oxygen concentration sensor provided in the exhaust passage at a location upstream of the exhaust emission control catalyst, to maximize the performance of the exhaust emission control catalyst, and to control the air-fuel ratio of an air-fuel mixture supplied to the engine in a range near a theoretical air-fuel ratio, based on the detected concentration of oxygen.
In a multi-cylinder engine in which the exhaust gas is discharged in sequence from a plurality of cylinders at every predetermined crank angle, the pulsation of the flow of exhaust gas in an exhaust passage is suppressed to a relatively small level. However, in a single-cylinder 4-cycle engine in which the exhaust stroke occurs only one time per two rotations of a crankshaft, a large pulsation may be produced in the flow of exhaust gas in the exhaust passage, and the exhaust gas passing through the exhaust emission control catalyst, may flow backwards in the exhaust passage due to the pulsation in some cases. The exhaust emission control catalyst has the effect of oxidizing the exhaust gas and for this reason, the concentration of oxygen in the exhaust gas flowing backwards from the exhaust emission control catalyst is deviated from an intrinsic value. If the false concentration of oxygen is detected for feedback-control of the amount of fuel supplied, there is a possibility that proper control cannot be carried out. Particularly, it is a recent tendency to position the exhaust port and the exhaust emission control catalyst at a smaller distance spaced apart from each other, in order to activate the exhaust emission control catalyst and provide compactness of the engine. Therefore, the distance between the exhaust emission control catalyst and the oxygen concentration sensor is also smaller, wherein the pulsation of the exhaust gas is liable to exert an influence.